1. Field of the Invention
The present invention generally relates to a gyro for detecting angular velocity, and more particularly to a tuning-fork type vibratory gyro using a piezoelectric substance and a detection circuit (sensor) which processes an output signal of the vibratory gyro.
A gyroscope has been used to identify the current position of a vehicle such as an airplane, a ship or a satellite. Recently, a gyroscope has been applied to devices for personal use, such as car navigation and vibration detection in video cameras and still cameras.
A piezoelectric vibratory gyro utilizes the effect in which when an angular velocity is applied to the gyro which is being vibrated, a Coriolis force is produced in a direction perpendicular to the direction in which the gyro is vibrated. Various types of piezoelectric vibratory gyro have been proposed. Recently, a tuning-fork type vibratory gyro has been attracted because it has relatively high cost performance. Particularly, there have been considerable activities in the development of a tuning-fork type vibratory gyro utilizing a piezoelectric single crystal.
Such tuning-fork type vibratory gyros utilizing a piezoelectric single crystal are proposed in, for example, U.S. Pat. Nos. 5,329,816 and 5,251,483. The U.S. Pat. No. 5,329,816 discloses a tuning-fork type vibratory gyro (gyro element) utilizing a piezoelectric single crystal shaped so that two arms and a base supporting the arms are integrally formed. Drive electrodes for driving a tuning-fork vibration are provided to one of the two arms, and detection electrodes are provided to the other arm in order to detect a voltage based on the angular velocity applied to the gyro.
FIGS. 1A and 1B show a tuning-fork type vibratory gyro having an electrode arrangement as described above. The gyro includes a tuning fork made of a piezoelectric single crystal, such as LiTaO.sub.3 or LiNbO.sub.3. The tuning fork has two arms 11 and 12, and a base 13 which integrally connects the arms 11 and 12 together. As shown in FIG. 1B, drive electrodes 14-17 are attached to the arm 11, and detection electrodes 18 and 19 are attached to the arm 12. A drive source 20, which generates a rectangular wave signal, is connected to the electrodes 14-17. A detection signal (voltage) depending on the angular velocity is output across the detection electrodes 18 and 19.
U.S. Pat. No. 5,251,483 has a piezoelectric tuning-fork type vibratory gyro having an electrode arrangement different from that of the gyro shown in FIGS. 1A and 1B. The gyro disclosed in U.S. Pat. No. 5,251,483 has detection electrodes attached to two arms. Generally, such an electrode arrangement is called a differential-type structure. The detection electrodes disclosed in U.S. Pat. No. 5,251,483 are provided to four or three surfaces of each of the two arms.
A vibratory gyro having the electrode arrangement as described above is shown in FIGS. 2A and 2B. Electrodes 14-17 and 25 are provided to the arm 11, and electrodes 21-24 and 26 are provided to the arm 12. The electrodes 15, 17, 21 and 23 function as drive electrodes, and electrodes 14, 16, 22, 24, 25 and 26 function as detection electrodes. As shown in FIG. 2B, two detection signals DET1 and DET2 are obtained, and the potential difference between the detection signals DET1 and DET2 corresponds to the angular velocity applied to the gyro.
FIG. 3 shows an operation of the gyros shown in FIGS. 1A, 1B, 2A and 2B. When the drive signal (voltage) generated by the drive source is applied to the drive electrodes, the two arms 11 and 12 are vibrated in the X directions. When the vibratory gyro is rotated about the Z axis, the two arms 11 and 12 are vibrated in the Y directions perpendicular to the X directions. The magnitude of the vibrations of the arms 11 and 12 in the Y directions is proportional to the Coriolis force, which is proportional to the angular velocity. Hence, a signal (a detection signal) proportional to the vibrations of the arms 11 and 12 in the Y directions reflect the value of the angular velocity applied to the gyro.
A detection circuit is provided which senses the detection signal. A detection circuit 27 shown in FIG. 4 is used for the vibratory gyro in which the drive electrodes are attached to one of the two arms 11 and 12 and the detection electrodes are provided to the other one of the arms 11 and 12. The detection signal from the vibratory gyro is applied to a synchronous detection circuit 31 via a phase adjustment circuit (not shown). The synchronous detection circuit 31 performs a synchronous detection in which a drive signal output by a drive circuit 30 is used as a reference signal for synchronous detection. A resultant signal derived from the synchronous detection circuit 31 is applied to a differential amplifier 32 via a smoothing circuit (not shown). The differential amplifier 32 performs a differential amplifying operation between the synchronous detection output and an offset voltage produced by an offset adjustment circuit 29 supplied with a power supply voltage 28. The above differential amplifying operation results in first and second output signals OUT1 and OUT2. The value of the voltage difference between the first output signal OUT1 and the second output signal OUT2 indicates the value of the angular velocity applied to the gyro, and the sign of the voltage difference indicates the direction of the rotation.
FIG. 5 shows a detection circuit 33 for use in the gyro having the electrode arrangement shown in FIGS. 2A and 2B. The detection signals DET1 and DET2 are applied to a differential amplifier 34, which performs a differential amplifying operation thereon. An output signal of the differential amplifier 34 is compared with the offset voltage by the differential amplifier circuit 32 used in the structure shown in FIG. 4. The differential amplifier 32 results in first and second output signals OUT1 and OUT2.
The detection signals DET1 and DET2 are subjected to the differential amplifying operation, so that leakage voltage which may be included in the detection signals DET1 and DET2 can be canceled. It should be noted that the gyro produces no detection signals if the gyro does not receive any angular velocity. However, the gyro may slightly produce detection signals irrespective of the gyro does not receive any angular velocity. Such detection signals are leakage voltages or signals.
FIG. 6 shows factors which cause the leakage voltage. The factors can be categorized in three groups. The first group of factors is called an electro-magnetic coupling leakage and is due to a surplus component of force coefficients caused by an unbalanced situation of the electrodes (errors in the size of electrodes and/or positions thereof). The electro-magnetic coupling leakage includes a leakage on the drive side and a leakage on the detection side. The second group of factors is called an electrostatic coupling leakage and is due to an electrostatic coupling capacitance between the input and output sides, that is, between the drive-side electrodes and the detection-side electrodes. The third group of factors is called a mechanical coupling leakage and is due to a mechanical coupling between the drive-side vibration and the detection-side vibration.
It may be possible to reduce the leakage voltages due to any of the first through third groups of factors by means of complex and troublesome works. For example, the electrodes are finely formed and finely positioned. If a positional error of an electrode happens, the electrode is cut off, for example. Alternatively or additionally, as shown in FIGS. 7A and 7B, a corner portion 35 of one or both of the arms 11 and 12 is cut off in order to change the moment of at least one of the arms 11 and 12 and thus reduce an unwanted vibration.
However, in practice, it is very difficult to greatly reduce the leakage, preferably to zero, whereas the above adjustment works are complex and troublesome. For example, in the gyro of the type shown in FIGS. 1A and 1B, the leakage voltage is, as shown in part (B) of FIG. 8, step portions in a sin wave of the detection signal which has the 90.degree. out-of-phase with the drive signal which is a continuous rectangular wave signal shown in part (A) of FIG. 8. The step portions are caused by the electrostatic coupling leakage. The sine wave of the detection signal is also caused by a leakage due to the electro-mechanical coupling leakage.
FIG. 9 shows the operation of the gyro of the type shown in FIGS. 2A and 2B. As shown in parts (B) and (C) of FIG. 9, the detection signals DET1 and DET2 include respective electrostatic coupling leakage components having different magnitudes and respective sin-wave leakage components having different magnitudes. A sine-wave component remains by the differential amplifying operation on the detection signals DET1 and DET2, while the electrostatic coupling leakages can be canceled, as shown in part (D) of FIG. 9. If it is attempted to cancel the sin-wave leakage components, an unwanted component due to the electrostatic coupling remains in the output signal of the differential amplifying operation. It can be seen from the above that a simple differential amplifying operation cannot completely eliminate the leakage components. Further, a leakage voltage obtained when the gyro does not receive any angular velocity degrades the resolution of the gyro.